1. Field of the Invention
This invention relates to impact-resistant laminates, and in particular to impact-resistant window panels which embody at least one glass sheet. Some such panels are bullet-resistant and resist bullets or other high energy impacts with little or no spalling from the rear face of the panel opposite to the face which receives the impact. Other such panels are "anti-vandal" panels which resist manual attack with weapons such as hammers or pick handles, and impacts of stones and bricks, but do not necessarily resist bullets.
2. Description of the Prior Art
Prior proposals for bullet-resistant glass laminates, and anti-vandal glass laminates usually either accepted the risk of spall from a rear glass face of the laminate or have included a rear plastics sheet, for example a thick polycarbonate or stretched acrylic sheet which is adhered to the rear glass face to combat spall.
In one example of use of the laminate as a vehicle window panel, the front sheet which receives the impact is a glass sheet and the rear sheet which is inside a vehicle is a thick polycarbonate sheet which is ductile and has good resistance to spalling. Impact-resistant laminates often comprise a number of glass sheets which are laminated together with a rear polycarbonate sheet which provides good resistance to spalling. GB 1 504 198 describes an impact resistant laminate comprising a number of glass sheets with a rear sheet made of polycarbonate having a brittle scratch-resistant coating on its exposed face. The tendency of the polycarbonate sheet to spall, due to some extent to the brittle nature of the protective layer, is said to be reduced by reducing the thickness of the polycarbonate sheet to a thickness in the range 0.76 to 5.6 mm, and the results given indicate that under certain conditions of "medium power" impact the laminate functions satisfactorily.
In GB 2 011 836 A there is a proposal for an improved impact resistant laminate, which resists a single shot, and which comprises a number of sheets of glass which are adhered together by means of transparent plastics interlayers which serve as the impact shock receiving layers of the laminate. These laminated glass sheets are mounted in a frame and are separated by an air space from a polycarbonate sheet 9.5 mm thick to which a scratch-resistant coat of self-healing polyurethane is applied.
GB 1 394 271 describes laminated glass windshields for vehicles. The windshield may comprise a glass sheet, a sheet of plastically deformable layer, for example polyvinylbutyral, adhered to the glass, and a composite structure adhered to the polyvinylbutyral. The composite structure may comprise a reinforcing sheet 0.1 to 0.2 mm thick, with a protective layer of self-healing polyurethane. A number of materials are suitable for the reinforcing sheet, for example an amorphous polyamide or copolyamide, a cellulose acetobutyrate or triacetate, a polycarbonate, an ionomer or a polyester. The total thickness of the wdndshield would be about 4mm.
FR 2181635 discloses the use of a polyurethane coating 0.75 mm to 3.8 mm thick, but preferably at least 1.27 mm thick, as anti-spall shield to prevent spall from glass or plastics, particularly polycarbonate. The polyurethane layer is protective cladding and accordingly requires a relatively large thickness of from 0.75 mm to 3.8 mm to act as an effective anti-spall shield.
EP 0130886 discloses bullet-proof glazing wherein a polycarbonate sheet is adhered to a rear surface of a glass sheet. The polycarbonate sheet preferably has a thickness of between 2 mm and 5 mm. A self-healing coating is adhered to the polycarbonate sheet by an adhesive coating of polyurethane. In preferred embodiments, the thickness of the self-healing coating is 0.5 mm when applied to polycarbonate sheet 2 mm thick.
GB 1142817 discloses a laminated safety pane comprising a layer of polycarbonate 0.5 to 2 mm thick adhered by a layer of clear adhesive 0.05 to 1 mm thick to a glass pane 2.5 to 7 mm thick. The polycarbonate is used to provide impact strength and is not used as an anti-spall layer. There is also no suggestion that the polycarbonate sheet could be effective as an anti-spall layer.
GB 1221285 discloses a vehicle windscreen having an inner sheet of plastics, which can be polycarbonate, which is of smaller overall dimensions than the glass enabling the windscreen to shear around its edge on head impact. The polycarbonate layer is used in place of a glass inner layer to stop the head of a passenger penetrating a glass layer on impact.
The use of a polycarbonate or stretched acrylic sheets as the rear impact resistant ply of a laminated window panel has added considerably to the cost of the laminate. In some cases the risk of spall from a rear glass sheet has been tolerated, rather than incurring the additional cost of such a rear ply.
The Inventor has found that a thin polycarbonate sheet having an outer self-healing coating can act as an effective spall-resistant rear lamina when adhered to a rear glass face of an impact-resistant laminate, in particular an impact-resistant window panel.
Further the Inventor has provided a solution to the problem of the effective application of a self-healing abrasion-resistant anti-spall shield to the rearmost glass face of a curved glass laminate, for example for use as impact resistant window panels for aircraft and railway locomotives, or as anti-vandal windows for automobiles.